amie question paper (electrical machines) · pdf fileajjswer five questions, ... salient-pole...

AMIE(I) STUDY CIRCLE, SECOND FLOOR, SULTAN TOWER, ROORKEE - 247667 (UTTARANCHAL) EMAIL: [email protected] Ph: (01332) 266328, 9412903929, 9897394020

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S'09: 5 AN: EL 405 (1466)

.ELECTRICAL MACHINES

Time : Three hours ·

Maximum Marks: 100

AJJswer FIVE questions, taking ANY TWO from Group A, ANY TWO from Group B and ALL from Group C.

AU parts of a.question (a, b, etc.) should be answered at one place.

Arlswer should be brief and to-the-point and be supplemented with n~t sketches. Unnecessary long answers

may result in Joss of marks.

Any missing or wrong data may be assumed suitably giving proper justification.

Figures on the .right -hand side margin indicate fuD marks.

Group A

1. (a ) Three d. c. shwnt dynamos, driven by a prime mover, each having a field resistance of 120 ohms and an armature resistance of 0·1 ohm, are connected to a i20V bus. These three generators have generated voltages of 125V, 120Vand 114 V; respectively. For each of the generators, calculate the (i ) line current drawn from or delivered to the bus and the armature current, and (ii) power drawn from or delivered to

· the bus and the power generated. 1 0

(b) Discuss factors determining the load distribution between a number of d.c. shunt dynamos running in parallel. 6

(Turn Over)



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(c) For d. c. compound generators operating in parallel,

explain why a diverter for given compound generator . cannot be connected in parallel with the series field

of that particular machine. 4

~. (a ) Derive expressions for the current shared by two transformers operating in parallel, with unequal no-load voltages. 5

( b ) Explain the method of estimating the load sharing when n number of transformers are operating in parallel. 5 ·

(c) Two transformers T1 andT2 ofdifferentkVAratings

but equal voltage ratings operate ill parallel and supply

a common load of 700 kVA at 0·8pJ. lag.

Transformer T1 has a rating of 500 kVA, resistive

drop of 1% and reactance drop of 5%. Transformer

T 2 has a rating of 250 kVA, resistive drop of 1·25%

and reactance drop of 4%. Calculate the load shared

by each transformer and the p.f. at which these are

working. 10

t (a) A 750 MVA, 13·8 kV, 50Hz star-connected,

three-phase, salient-pole, synchronous generator has

direct axis synchronous reactance (Xd) 1·83 ohm and

quadrature axis synchronous reactance (Xq) 1 ·21

ohm. It delivers the rated load at 0·8 p.f. lagging.

Neglect the annature resistance. Determine the

U ) voltage regulation, and ( ii ) power developed by

the generator. ·1 0

(b) Why is the direct axis synchronous reactance larger than the quadrature axis synchronous reactance in a salient-pole alternator?. 4

'09:5AN:EL 405 (1466) ( 2 ) (Continued)

{ c ) Explai:Q the nature of external characteristics of a

synchronous geperator for unity, leading, and lagging power factors. 6 ·

4. (a) Al20V, 50Hz, three-phase; star-connected, roundrotor synchronous motor has an armature-winding resistance of ( Q,S + j 3} ohm/phase. The motor takes a current of lOA at 0·8 pL leading when operating at a certain field current. The load is gr:adually increased until· the motor develops the maximum torque while the field current is held constant. Determine the new line current, ·the power·factor, and the torque develope4 by the motor. 10

( b ) The· armature current increases ·with an increase in the field current of a synchronous motor. What is the initial power-factor angle? What is the effect on the torque-angle? 6

( c ) Why is it necessary to improve the overall power factor of a manufacturing plant 'i 4

GroupB

5. (a ) A 230 V, 50Hz,4-pole, delta-connected three-phase induction motor operates at a full-load speed of 1440 rpm. The power developed at this speed is 2 H.P. and the rotor current is 4·5 amp. If the supply voltage fluCtuates ±10%, . determine (i) torque range, and ( ii ) current range. 1 0

(b) Explain the method of speed control of 3-phase induction motor by varying the supply frequency. 5 .

(c) What is the purpose of using deep-bar cage rotors? · Explain the construction and working of a deep-bar cage motor. 5

I 1 l ( 'T--- r\ .. ...:._,



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6. (a) A 208V; three-phase, six-pole, 50Hz, 7·5H.P. · \YOund rotor induction inotor has both its stator and · rotor connected in star. The rotor has half the number· of tunis the stator has. Full-load (rated) speed is 1125 rpm. The rotor resistance is 0·08 ohms/phase,and the locked rotorreactance is 0·4 ohm/phase. Calculate

. {i) locked-rotor voltage per phase, (ii) rotor current per phase at rated speed, {iii) ratedrotor power input per phase, (iv ) rated rotor copper loss per phase,

· { v) rotor ·power developed per· phase in h.p., ( vi ) rated t()rque developed per phase, { vii ) total rotor torque deveioped, ( viii ) total output rotor torque. 12

( b ) Draw and explain the power flow diagram of a 3-phase induction motor. Show that rotor-copper loss is slip times the power input to rotor. 4

(c) Cite possible reasons why a three-phase induction motor fails to start. 4

7. (a) A 230 V, 50 Hz, 4-pole single-phase induction motor has the following equivalent circuit impedances: R 111, (resistanceofthemainstatorwinding) = 2·2ohm

xlm (leakagereactanceofthemainstatorwinding) = 3·1 ohm

R '2 (standstill rotor resistance referred to the main stator winding)= 4·5 ohm

X' 2 (standstill rotor leakage reactance ref erred to the main stator winding)= 2·6 ohm

XM (magnetizing reactance)= 80 ohm:·

Friction, windage and core loss = 40 W, for. a slip of 0·03p.u.

S'09: SAN: EL 40S ( 1466) ( 4 ) (Continued)

Calculate · (j) input current, ( il) vower factor, (iii) developed power, (iv) output power, and ( v) efficiency. 14

(b) Explain why a universal motor can operate from d.c. as well as a. c. supply. What are the main differences in construction between a.c. series motor and d.c . senesmotor? 3+3

8. (a) A 0·5 h.p., 230 V~ single-phase induction motor (split-phase) takes a current of 4-2 amp lagging the voltage by 10° for the a·uxiliary winding and a current of 6·2 amp lagging the voltage by 40° for its main winding. Find (i) total currept and p.f. at the time of starting, ( ii) total p.f. and current during running, and (iii) phase angle between the main winding current and auxiliary winding current, ( v) power drawn by main winding, ( vi ) total power drawn during starting, ( vii) total power drawn during running, ( viff) efficiency of the motor if the motor output is 0·5 h.p. (metric) and curents are as above. 12

( b ) . Name three types of d. c. stepping motors. What constructional differences are found to distinguish one type from another? For the same number of poles (rotor), which d.c. stepper produces the largest and smallest stepping angles, respectively. · 8

Groupe

9. Answer the following in two or three sentences: 10 x2

( i) Why cannot an induction motor run at synchronous speed?

( ii) Distinguish b~tween harmonic induction torque and harmonic synchronous torque developed in a 3-phase induction motor.

S'09: SAN: EL 405 (1466) ( s ) (TurnOver)



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AMIE(I) STUDY CIRCLE, SECOND FLOOR, SULTAN TOWER, ROORKEE - 247667 (UTTARANCHAL) EMAIL: [email protected] Ph: (01332) 266328, 9412903929


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AMIE(I) STUDY CIRCLE, IInd FLOOR, SULTAN TOWER, ROORKEE - 247667 (UTTARANCHAL) EMAIL: [email protected] Ph: (01332) 266328, 9412903929 Web: www.amiestudycircle.com


12

S'10: SAN: EL405 (1466)

ELECTRICAL MACHINES

Time : Three hours

Maximum Marks : 100

Answer FIVE questions, taking ANY TWO from Group A, ANY TWO from Group B and ALL from Group C.

All parts of a question ( a, b, etc. ) should be answered at one place.

Answer should be brief and to-the-point and be supplemented with neat sketches. Unnecessary long answers

may result in loss of marks.


Figures on the right-hand side margin indicate full marks.

Group A

1. (a) Draw the speed torque characteristics of a d.c. shunt motor whose speed is controlled through field current. 5

(b) A 250 V d.c. shunt motor hasa field resistance of 125 ohms and an armature resistance of 1 ohm. It takes an armature current of 25A at a speed of 900 r.p.m. It is required to increase the speed to 1100 r.p.m. keeping the torque constant. Find the value of additional resistance to be added in the field circuit to get this speed. Assume unsaturated magnetic circuit. 15



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2. (a) What are the distinguishing features of star -star, star- (b) A slip ring induction motor has a full load speed of

delta, delta-star and delta-delta connections in 3- 288 r.p.m. and frequency is 50 Hz.· Find the

phase transformers ? Compare their advantages (i) number of poles and slip, and (ii) slip for full

and disadvantages. 12 load, if rotor resistance is doubled. If initial rotor copper loss is 275 W, find new value of rotor

(b) Explain the Scott connection giving a suitable copper loss. 2x4

diagram. Show the currents in windings in the (c) List the methods for speed control of 3-phase diagram. 8 induction moror. 4

3. (a) What do you understand by direct and quadrature 6. The results of no-load and blocked rotor tests on a axis reactances of synchronous generators ? 400 V, 50 h.p., 50 Hz, 3-phase, 4-pole induction motor Discuss them in detail. 8 are as under : No-load test, V0 = 400 V, 1

0 = 30 A, ~ =

1800 W, blocked rotor test, V,c = 110 V, I.e = 80 A,

(b) How can direct and quadrature axes reactances W =4000W. .., be measured ? Explain. 6 The stator is star-connected having a resistance

of 0.1 ohm per phase. Draw the circle diagram and

(c) Explain briefly any one method for synchroni- find (a) full load current, power factor, slip, torque, and

sation of 3-phase synchronous generators. 6 efficiency, . (b) starting torque, maximum torque and maximum output 10 + 10

4. (a) Draw V curves for a synchronous motor. Justify 7. {a~ Briefly discuss the principle of split phase, shaded the shape of these curves. 8 pole artd capacitor single-phase induction motor. 3x3

(b) What is the application of a synchronous con-6

(b) Discuss the principle ofa.c. series motor. Draw denser ? Explain. its performance characteristics. 3+3

(c) What is the significance of torque-angle charac'- (c) What are the applications of a.c. series motor? 5 teristics of a synchronous machine ? Explain. 6

8. Write short notes on the following : 7+6+7

Group B (a) Repulsionmotor

5. (a) Draw arid explain the torque-slip curve of a (br) d.G. servomotor

3-phase induction motor. 8 (c) -Pun .:.out torque ofinduction motor.



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Groupe

9. Select the best alternative for the following : 20 X I

(i) Two d.c. shunt generators are operating in parallel. It is desired to shutdown one of the generators. The proper method is to

(a) open its switch suddenly. (b) open its switch slowly. (c) reduce its excitation to zero. (d) Any one of the above.

(ii) The torque developed by a d.c. motor is proportional to

(a) flux. (b) armature current. (c) flux and armature current. (d) None of the above~

(iii) The resistance of shunt fieid is about

(a) lOOohm (b) 10ohm (c) 2ohm (d) 0.2ohm

(iv) Suribume test can be used only for

(a) series motor. (b) shunt motor. (c) shunt and series motor. (d) shunt afiocompound motor.

( v) When the secondary winding of a transfomer is short-circuited, the power factor at primary terminals is about

(a) 0.2lagging (b) 0.2leading (c) 0.8lagging (d) 0.8leading

(vi) The secondary winding of a transformer is opencircuited. The· primary is connected to a d.c. voltage equal to rated primary voltage. The primary current would be about

(a) 50p.u. (b) 10p.u. (c) 1 p.u. (d) O.lp.u.

(vii) During short-circuit test on transformer, iron . losses are negligible because

(a) current is high. (b) mutual flux is small. (c) frequency is low. (d) power factor is low.

(viii) Under short-circuit conditions, the power factor· of a synchronous machine is

(a) 1 (b) about 0.8 (c) almost zero lagging (d) about 0.5



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(ix) In modern large size synchronous generators, synchronous impedance.is about

(a) 0.2p.u.

(b) O.Sp.u.

(c) 1.0 p.u.

(d) 0.05 p.u.

(x) In two reaotiian theory, the armature current is d-sed into I and I. IfE and V are gene--~- d q . . .

rated-e.m.:C. and terminal voltage, then

(a) ld is perpelldicular to V ~d I9

is in phase wdhY.

(b) ·1. is in .phase wiltlb.· E ·and 19

is _perpendicular toE.

(c) Id is perpendicular to E and 19

is . in phase

withE.

(d) Id is. in phase with V and 19

is perpendicular

toV.

(xi) When the real pc)wer.output of a cylindrical rotor - alternator is maximum, the reactive power output is

(a) 3 V21X. · (b)- 3 V21X. (c) 3EV IX. (d) J£2/X.

(xii) An under-excited synchronous motor operates. at

(a) laggingp.f. (b) unityp.f. (c) leadingp.f. (d) Aity one oftheabove.

(xiii) For fixed load on shaft, the armature current of a synchronous motor is minimum when p.f is

(a) -less than 1 and leading. (b) unity. (c) lagging. (d) leading or lagging.

(xiv) In a synchronous motor,

(a) E is always less than V. (b) E= V. (c) E is always more than V. (d) E may be more orless than V. -

(xv) At s = 0, torque of a ~-phase induction motor is

(a) 0 (b) maximum. (c) very high. (d) nearly zero.

(xvi) The speed change of a 3-phase induction motor from no-load to full-load is about

·(a) 40% (b) 20%

-(c) 10% (d) 3%



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W'10: 5 AN: El405 (1466)

ELECTRICAL MACHINES

Time : Three hours

Maximum Marks : 100

Answer FNE questions, taking ANY TWO .trom Group A, ANY 1WO from Group B ;md AlL from Group C.

All parts of a question (a, b, etc.) should be answered at one place.

Answer should be brief and to-Lhe-point and be supplemented with neat sketches. Unnecessary long answers

may result in Joss of marks.



Group A

1. (a) What are necessary conditions for parallel opera-tion of d. c. generators? 4

(b) The full-load regulation of two d. c. shunt generators are 5.5% and 4.5%. 11u~. rating of both the machines are 250 kW and 450 V. How will they share a load current of 900 A ? Determine the terminal voltage and currents shared by machines, if the load resistance is 0.55 Q. 8

(c) A d.c. shunt motor has the following data: Rated voltage = 2ZO V Speed= 1050 r.p.m. Armature resistance = 0.4 'Q

Calculate the value of resistance required to be inserted in the armature circuit to reduce the speed to 850 r.p.m. at an armature current of 22 A. 8



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2. (a) Explain the difference between Y d 1 and Dy 11 of a three-ph.ase transfom1er. 4

(b) Draw the electrical connections and vector diagrams of (i) delta/zig-zag of three-phase transformer, and (ii) double delta of three-phase transformer. 4+4

(c) Two electric furnaces are connected with the help of two scott-connected transfqrmers. The threephase supply was made to this transformer with the help of a 15 kV source. The load on one transformer is 650 kW at 120V and the load on the other transformer is 1050 kW at 120V.Ifthe power factor is unity, determine the current flow in all three-phase lines. 8

3. (a ) How do you determine the regulation of synchro-nous generator by the Potier method ? 5

(b) A three-phase star-connected synchronous generator has effective resistance and synchronous reactance of 1.6 Q and 35 Q, respectively per phase. Determine the percentage regulation for a load of 1320 kW at power factors of (i) 0.8 lagging, and ( ii ) 0. 8 leading when the rating of the machine is 1650 kVA and 13.8 kV. 7

(c) A synchronous generator has the following data: Direct axis synchronous reactance=

0.85 p.u. Quadrature axis synchronous reactance =

0.55 p.u. Load is full-load at lagging power factor=

0.8 Determine per unit value of the open-circuit voltage with full-load excitation. Assume that the resistance and the effect of saturation are neglected. 8

4. (a ) Explain the V -curves of synchrono~s machine. 5

( b J A three-phase star- connected synchronous generator is of salient pole-type. It is being run at synchronous speed with field circuit open. ·n1e stator is supplied from a three-phase balanced supply. A voltmeter, which is connected across the line, provides the maximum and minimum readings as 2825 V and 2805 V, respectively. The line current varies from 3 60 A to 280 A. Determine the direct and quadrature axis synchronous reactance per phase. The armature resistance may be neg-lected. 8

(c) Determine the rating of the synchronous condenser, which will raise the power factor of the substation from 0. 7 lagging to 0.85 lagging. The substation is operating at full load of 1050 kVA and at a power factor of 0. 7 lagging. 7

Group B

5. (a) Explain the circle diagram of a three-phase in-duction motor. 5

(b) The test results of 400 V, 3. 7 kW, delta -connected induction motor are the follo\\ing:

No load test: 400 W, 2.8 a power factor= 0.25

Blocked rotor test: 200 V, 11.8 a power factor= 0.45

Draw the circle diagram and determine the ( i ) full load current; (ii) full load power factor; (iii) starting torque in terms of the full load torque at the rated normal condition. Consider that the copper losses will be both for the stator and the rotor. 8



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(c) Determ~ne the tapping on an auto transformer to start the three-phase induction motor at 40% of full-load torque. The short-circuit current is four times the full load current of the motor. Consider the rated slip of the motor 5%. Also, determine the current to be drawn from the supply as a percentage of the full-load current. 7

6. (a ) Describe a closed loop feedback system of speed control of three-phase slip ring induction motor. 4

{ b ) Two three-phase 50 Hz induction motors have 8 poles and 6 poles, respectively.· They are connected in cascade. The machine having 8 no. of poles is slip ring induction motor. Determine the frequency of the rotor currents and the slips being referred to each statoi iOtating field when the combined set has a slip of2.5% from the cascaded speed. 8

(c) A three-phase slip ring induction motor is provided with the following data :

( i) Maximum and minimum currents at the time of starting are kept at certain fixed values.

(ii) Full-load rated slip= 0.017

(iii) Rotor resistance per phase= 0.014 Q.

There are five branches of external resistances being connected to the rotor. Determine the resis-tance iQ. each branch. 8

7. (a ) What is capacitor split phase motor? Explain clearly its principle of operation. 6

(b) The inductive reactance and resistance of each winding of a split phase induction motor are 240 Q

and 82 Q, respectively. In series with one winding, there exist an additional resistor and a condenser. Determine the value of the additional resistance and capacitance of condenser to provide the same current in each winding with a phase difference of 90°. Assume the frequency of supply to be 50 fu: 8

( c ) What is the difference between repulsion induction motor and repulsion start induction motor? Explain clearly. 6

8. {a) How many types of d. c. servomotors are ther~? Describe them. What is drug cup induction motor? Explain clearly. 10

(b) A 115 V, 260 W, 5000 r.p.m., 50 Hz a.c. series motor is provided with the following data:

Total resistance= 1.6 Q

Total reactance= 16 Q

Stray power loss = 22 W Determine the (i) current and the powerfactor at the normal rated condition, ( ii) efficiency, speed and power factor when the load current is reduced to three-eighth of.the rated condition. 10

Groupe

9. Answer the following in brief: lOx 2

(i) What is the main problem of parallel operation of compound d.c. generators?

( ii) What are the advantages of speed control of d. c. motor by semiconductor converter?



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AMIE(I) STUDY CIRCLE, SECOND FLOOR, SULTAN TOWER, ROORKEE - 247667 (UTTARAKHAND) EMAIL: [email protected] Ph: (01332) 266328 Web: www.amiestudycircle.com


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S'11: 5 AN: EL 405(1466)

ELECfRICAL MACIDNES

Time : Three hours

Maximum Marks: 100

Answer FIVE questions, taking ANY TWO from Group A, A."<Y TWO from Group B and AIL from Group C.

All parts of a question (a, b, etc.) should be answered at one place.

Answer should be bdef and to-the-point and be supplemented with neat sketches. Unnecessary long answers

may result in loss of marks.


Figures on the dght-hand side margin indicate full marks.

Group A

1. (a ) Which method will you adopt to control the speed of a d.c. shunt motor below the base speed ? This method of speed control is called a constant torque drive method. Explain. 10

(b) The speed of a 500 V d.c. shunt motor is raised from 700 rpm to 1000 rpm by field weakening, the total torque remaining unchanged. The arma-ture and field resistances are 0.8Q and 750Q, respectively, and the current at the lower speed is 12 A. Calculate the additional shunt field resistance required, assuming the magnetic circuit to be unsaturated. Neglect all losses. l 0



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2. (a ) The Hopkinson test on two shunt machines gave the following results for full load: Line voltage = 250 V ; Line current =. excluding field currents, 50 A; Motor armature current=. 380A; Field currents= SA and 4.2A. Draw the circuit diagram and mark the values. Assuming resistance of each machine as 0.020, determine the efficiency of each machine. 8

( b ) A 200 V, 2000 rpm, 10 A, separately excited d.c. motor has an armature resistance is of 2 Q. Rated d.c. voltage is applied to both the armature and field winding of the motor. If the armature draws 5 A from the source. find the torque developed by the motor. 6

(c} A250kWand 750kW, 550Vgeneratorsoperate in parallel; delivering a total load of 1500 A The voltage regulation of the smaller machine is 0.058 in per unit and for larger one is 0.035 in per unit. Assume that. the external characteristics are straight lines. Effect of armature reaction is negligible. Determine the ( i ) current delivered by each machine, and ( ii) terminal voltage. 6·

3. (a) What are the requirements for a satisfactory para-llel operation of two 3-phase transformers? 4

( b ) A 100 kV A transformer, having 1% resistan~e and 4% leakage reactance, is operated in parallel with a 200 kVA transformer having 1% resistance and 6% leakage reactance. If the total load delivered is 300 kVA at unity p.f.; calculate the kVA load

on each transformer as well as the operating p.f. of each ~transfermer. Also~ -determine the largest value of load that can be delivered 'by the parallel combination of two transformers without overloading any of them. Derive the formula used. 16

4. (a ) Derive an expression for the power developed by a salient pole synchronous machine. 4

(b) What are the V -curves of a synchronous motor? 4

(c) A 3-phase, 5 kVA, 220V, 50 Hz star-connected alternator has negligible armature resistance and a synchronous reactance of 1 OQ per phase and is operated in parallel with a 3-phase 220V, 50 Hz supply. The machine is delivering rated kVA at 0.8 lagging p.f. If the field excitation is incr~ased by. 20%, wit.'tout changing t'le· prime mover power, find the new value of armature current and p.f. At what power output, would the alternator break from synchronism with this new value of excitation ? What are the corresponding values of armature current and p.f.? 12

GroupB

S. (a ) Name the various methods of speed control slip-ring and cage rotor induction motor. 4

(b) Two induction motors, with 4-poles and 6-poles, respectively, are connected in cumulative cascade. The frequency in the secondary circuit of 6-pole motor is observed to be 2.0 Hz. Determine the combined speed of the set and the slip in each machine. Assume supply frequency as 50 Hz. 8



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(c) An 8-pole, 50 Hz, 3-phase induction motor has an equivalent rotor resistance of 0.071 Q per phase. If its stalling speed is 630 rpm, how much resistance must be included per phase to obtain maximum torque at starting. Ignore magnetizing current.

6. (a ) Give two applications of servo motors.

(b) A 400V, 3-phase, 50 Hz, star-connected, induction motor has the following test results :

No-load test: 400V, 8.5A, llOOW Blocked rotor test: 180V, 45A, 5700W

Calculate the line current and power factor when operating at 4% slip. The stator resistance per phase

8

2

is 0.5 0. Give a proof of the slip scale method. 18

7. (a) A 3-phase, 50 Hz, 400V wound rotor induction motor runs at 960 rpm at full load. The rotor resistance and stand-still reactance per phase are 0.2 0 and 1.0 0, respectively. If a resistance of 1.8 0 is added to each phase of the rotor at standstill, what would be the ratio of starting torque with full voltage and the added resistance to the full load torque under normal running conditions ? State assumptions made in your calculations. Can

·the same starting torque be obtained with another value of additional resistance? Explain. If the answer is yes, find its value. 10

(b) A cage induction motor takes 175% of full-load current and develops 35% of full-load torque at starting, when operated with a star I delta starter. What will be the starting torque and current, if an. auto-transformer starter with 80% tapping is employed. 10

S'll :5AN:EL405 (1466) ( 4 ) (Continued)

8. (a) Why does a single-phase induction motor not-. possess any starting torque? 5

(b) What is a servo motor? Why and where is a drag-cup rotor servo motor used ? 5

(c) A 4-pole, 230V, 50Hz, single phase induction motor runs at 1425 rpm. The power absorbed by the forward and backward fields are 245 W and 35 W, respectively. The no load rotational losses is 45 W. Determine the developed torque and shaft torque. 10

Groupe

9. Answer the following in brief: 10x2

(i) Why is in a normal synchronous machine xq smaller than x d ?

( ii) Resistance of dampers for synchronous generators is lower than that of dampers for synchronous motors. Why?

(iii) What is meant by an infinite bus?

(iv) Distribution transformers are designed for lower iron losses. Why?

( v) What are the functions of transformer oil ?

( vi) Do we use laminations for all iron parts of an electrical machine. If not, why ?

( vii) Explain why in an induction motor a high value of rotor resistance is preferred at starting ? .

(viii) Why is an ordinary induction motor not started at normal voltage ?

g 11 . 5 AN. EI. 405 (I 46!') { < ' . {Tum Over!



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25

W'11 :5 AN:El405(1466)

ELECTRICAL MACHINES

Time: Three hours

Maximum Mark.s : 100

Answer FIVE questions, taking ANY TWO from Group A~ ANY TWO from Group B and ALL from Group C.

All parts (~l a question ( a, b, etc. ) should be answered at one place.

Answer should be brief and to-the-point and be supplemented with neat sketches. Unnecessary long answers may

result in loss of marks-.



Group A

i. (a) What is armature reaction? Describe demagnetizing and cross-magnetizing effects of armature reaction in a d. c. machine. 6

(b) A d.c. generator has an armature emf of 100 V when the useful flux per pole is 20 m Wb and speed is 800 rpm. Calculate the generated emf ( i) with the same flux and a speed of 1000 rpm, and (ii) which p. . flux per pole of24 mWb and a speed of900 rpm. 8

(b) Explain the process ofbuilding up of voltage in d.c. generators and give the conditions to be satisfied for voltage build-up. 6



26

2. (a) Explain the speed-current and speed-torque charac-(h) Discuss the phenomenon of sudden three-phase teristics of a d.c. series motor. Account for the short circuit at the armature terminals of an alter-shape of these characteristics curves. 6 nator. Draw a typical wave shape of curre~t and

(b) A shunt generator delivers 50 kW at 250 V when mark the different regions. Write an expressiOn of 6 the current. running at 400 rpm. The armature and field resis-

tance are 0.02 Q and 50 n, respectively. Calculate (c) What is a short-circuit ratio ? Why are modem speed of the machine when running as a shunt alternators designed with high value of short-circuit

4 motor and taking 50 kW input at 250 V. Allow 1 V ratio (SCR)? per brush for contact drop. 10

(c) Explain why a d.c. series motor should never run GroupB

Explain the principle of operation of a ~hree-phase unloaded. 4 5. (a) synchronous motor. Draw phasor diagrams. of

3. (a) State the conditions for proper operation of two cylinderical rotor synchronous moto~ for laggmg transformers in parallel giving reasons for imposition

6 and leading power factors, and discuss these

8 of these conditions. phasor diagrams.

(b) Two single phase transformers, having the same (b) Why is synchronous motor not self-starting ? What voltage ratio on no-load, operate in parallel to ::trP thP mPth()ck P't':nPr::tllvm:ecl to start svnchronous _ _.._, ...... ~- .......... __. ...... ____ ~ o-- ... ------J ------ .,;

2 supply a load of 1000 kVA at 0.8 power factor motors? lagging. One of the transformers is rated at 400

A three-phase, 3.3 kV, 2 pole, 3000 rpm, 934 kW k VA and has a per unit equivalent impedance of (c) (0.01 + j 0.06); and the other one is rated at 600 synchronous motor has an effici~nc.y of 0.~5 ~.u. kVA and has a per unit equivalent impedance of and delivers full-load torque With Its excitatiOn (0.01 + j 0.05). Determine the load on ~ach of the adjusted so that input power factor _is unity._ The transformer (in kVA) and the operatmg power moment of inertia of the motor and Its load 1s 30 factor. 10 kg-m2 and synchronous impedance is (0.0 +_i 11_. 1)

Q. Determine the period of undamped oscillatiOn (c) What is an open-delta (V-V) system ? What are on full load for small changer in load angle. 10

the applications of this system? 4

A three-phase, 16-pole synchronous generator h~s 6. (a) A 6-pole, 230 V, 50 Hz, star-connected, three-4. (a)

phase induction motor has the following parameters a resultant air-gap flux of0.06 Wb/pole. The flux 1s on a per-phase basis; R I = 0.5 n, R2 = 0.25 n, distributed simultaneously over the pole. The stator

has two slots per pole per phase and four conduc- x, = 0.75 n, x2 = 0.5 n, xo = 100 n, an_d tors per slot are accommodated in two layers. The Ro = 500 n. The friction and windage_ loss IS coil span is 150° electrical. Calculate this phase and 150 W. Find the efficiency of the motor at Its rated line-induced voltages when the machine runs at 375 slip of2.5 percent. 10 Ipm. 10



27



28

5'12:5 AN: EL 405 (1466)

ELECTRICAL MACHINES

Tzme : Three hours

Maximum Marks : 100

Answer FIVE questions, taking ANYTWOfrom Group A, At-NT\VO from Group B and ALL from Group C.

All parts of a question ( a, b, etc. ) should be answered at one place.

Answer should be brief and to-the-point and be supplemented with neat sketches. Unnecessary long answer may

result in loss of marks.


Figures on the right-hand side margin indicate full marks

Group A

1. (a) Which method will you adopt to control the speed of a d. c. shunt motor above the base speed? This method of speed control is called a constant power drive method. Explain. 10

(b) The magnetisation characteristic of a 4-pole d. c. series motor may be taken as proportional to the current over a part of working range ; on this basis the flux per pole is 4.5 m Wb/ A. The load requires a gross torque proportional to the square of the speed equal to 30 N .mat 1000 rpm. The armature is wave wound and has 492 armature conductors. Determine the speed at which the motor will run and the current it will draw when connected to a 220 V supply, the total resistance of the motor being 2.0 n. 10

(Turn Over)



29

2. (a) The Hopkinson test on two shunt machines gave 4. (a) Derive an expression for the power developed by a the following results for full load: Line voltage-250

salient pole synchronous machine. 4 V; line current, excluding field currents, 50 A ;mo-tor armature current, 380 A; and field currents 5 A

(b) What are the V-curves of a synchronous motor? 4 and 4.2 A. Draw the circuit diagram and mark the values. Assuming resistance of each machine as

8 (c) A 3-phase, 5 kVA, 220 V, 50 Hz star-connected 0.02 n, determine the efficiency of each machine. alternator has negligible armature resistance and a

A 200 V, 2000 rpm, 10 A, separately excited de synchronous reactance of 10 n per phase and is

(b) operated in parallel with a three-phase 220 V, 50 Hz motor has an armature resistance of2 n . ;Rated supply. The machine is deliveringratedkVAat0.8 d.c. voltage is applied to both the armature and lagging p.f. If the field excitation is increased by field winding of the motor. If the armature draws 20%, without changing the prime mover power, find 5 A from the source, fmd the torque developed

6 the new value of armature current and p.f. At what

by the motor. power output, would the alternator bre~ from synchronism with this new value of exc1tat10n ?

(c) A 250 kW and 750 kW, 550 V generators ope- What are the corresponding values of armature rate .in parallel, delivering a total load of 1500 A. current and p.f. ? 12 The voltage regulation of the smaller machine is 0.058 in per unit and for larger one is 0.035. GroupB Assume the external characteristic are straight lines. Effect of armature reaction is negligible. Determine s. (a) Explain why the rotor of a squirrel cage induction the (a) current delivered by each machine, and (b) motor is forced to rotate in the same direction as terminal voltage. 6 the rotating magnetic field 5

3. (a) What are the requirements for a satisfactory para- (b) What are two types of rotor constructions used in llel operation of two 3-phase transformers? 4 induction motors? Compare them. 5

(b) A l 00 k VA transformer having 1% resistance and (c) A balanced three-phase induction motor ha.., an effici-4% leakage reactance is operated in parallel with ency of85% when its output is 45 kW. At this load,

both the star copper loss and rotor copper loss are a 200 kVA transformer having 1% resistance and equal to the core loss. The mechanical losses are . 6% leakge reactance. If the total load delivered is one·-fourth of the no-load losses. Calculate the slip. 10 300 kVA at unity p.f., calculate the k VA load on

each transformer as well as the operating p.f. of 6. (a) Explain how the speed of slip-ring induction motor each transformer. Also, detennine the largest value can be varied by varying its rotor circuit resistance. ofload that can be delivered by the parallel combi- What are the limitations of this method? 10 nation of two. transformers withoutoverloading any of them. Derive the formula used. 16 (b) A 8-pole 50 Hz, three-phase induction motor is loaded

2: 5 AN :EL 405 (1466) { 2 ) ( Continued ) S'l2: 5 AN :EL 405 (1466) ( 3 ) (Turn Over)



30

7. (a)

deliberately to a point where pull-out torque will occur. The rotor resistance per phase is 0.3 n and m otorstaJlat650 :rpm .c aJcu:Jate the 'l) breakdown slip, (iz) locked rotor reactance, (iii) rotor frequency at the maximum torque point, and (iv) suppose this motor develops twice the full-load torque when started with rotor short-cirCuited and it runs at a fullload speed of 675 rpm. If resistance of 0. 7 n is added in series with rotor per phase, calculate the new full-load with added resistance. 10

A three-phase, 50 Hz, 400 V wound rotor induction motor runs at 960 rpm at full load. The rotor resistance and standstill reactance per phase are 0.2 n and 1.0 n , respectively: If a resistance of 1.8 n is added to eacli phase of the rotor at standstill, what would be the ratio of starting torque with full voltage and the added resistance to the full load torque under normal running conditions? State asswnptions made in your calculations. Can the same starting torque be obtained with another value of additional resistance? Explain. If the answer is yes, find its value. - - 10

(b) A cage induction motor takes 175% of full-load current and develops 35% of full-load torque at starting, when operated with a star/delta starter. What will be the starting torque and current, if an auto-transformer starter with 80% tapping is em-ployed? 10

8. (a) Why does a single-phase induction motor not possess any starting torque ? 5

(b) What is a servomotor? Why and where is a drag-cup rotor servomotor used ? 5

(c) A 4-pole, 230 V, 50 Hz, single phase induction motor runs at 1425 rpm. The power absorbed by the forward and backward fields are 245 W and 35 W, respectively. The no-load rotational losses is 45 W. Determine the developed torque and shaft torque. 10

S'l2: 5 AN :EL405 (1466) ( 4) ( Continued)

Groupe

9. Choosethecorrectanswerforthefollowing: 20 x 1

(z) If a 230 V d. c. series motor is connected to 230 V a.c. supply, the

(a) motor will VIbrate violently.

(b) motor will run with less efficiency and more sparking.

(c) motor will not run.

(d) fuse will be blown.

(iz) The direction of rotation of a stngle-phase indue-. tion motor can be reversed by,reversing the

(a) leads of main winding.

(b) leads of auxiliru:ywinding.

(c) supplyleads.

(d) either(a)or(b)above.

(iiz) For which one of the iDllowing applications is the universalmotormost suital>le?

(a) Tablefan

(b) Tape recorder

(c) Hand-operated drilling machine

(d) Recorder player

(iv) Direct axis synchronous reactance of a salient pole synchronous motor can be estimated fairly from

S'l2: 5 AN :EL405 (1466) ( 5 ) (Turn Over)



31

short-circuit and open-circuit tests.

~)True

(b) False

( v) In a three-phase round-rotor synchronous machine, the effect of armature reaction is taken care of by

(a) synchronous reactance.

(b) magnetizing reactance.

(c) self-reactance.

(d) leakage reactance.

(vi) The sum of armature mmfs in three-phases of the stator winding of a synchronous generator is zero.

(a) True

(b) False

(vii) A three-phase synchronous motor, having constant excitation, drives a load taking power from infmite bus at leading p.f., if the shaft load decreases,

(a) thepowerangledecreases whilep.f. increases.

(b) the power angle increases while p.f. decreases.

(c) both power angle and p.f. decrease.

(d) both power angle and p.f. increase.

(viii) For the same power rating, an alternator operating at lower voltage will be

(a) less noisy.

(b) costlier.

(c) larger in size.

(d) more efficient.

( ix) In a synchronous generator, a divided winding rotor is preferable to a conventional winding rotor because of

(a) higherefficiency.

(b) increased steady-state stability limit.

(c) higher short circuit ratio.

(d) better damping.

(x) An induction motor and a synchronous motor are connected to a common feeder line. To.operate the feeder line at unity p.f., the synchronous motor should operate · .

(a) underexcited.

(b) overexcited.

(c) nonnally.excited.

(d) disconnected from common terminals.

(xi) It is desired to eliminate fifth hannonic voltage from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of

(a) 30°

S' 12: 5 AN :EL 405 (1466) ( 7 ) (Turn Over)



32

(xii) High frequency transformers use

(a )aluminium core.

(b) ironcore.

(c) air core.

(d) coppercore.

(xiii) The all day efficiency of a transformer is

(a) less than its power efficiency.

(b) equal to power efficiency.

(c) more than pow~ciency.

(d) unrelated to power efficiency.

(xiv )'Circular coil sections araused, because they

(a) havebettermecbanieilstrength.

(b) are easyto wind.

{c) reducecopperloss.

(tlj reduce ironJoss.

(xv) In au aut(i).-tranSf(!)rmer, power iis transferra!i through the

(a) inductive:process.

(b) conduction process.

(c) mutual coupling process.

! :5 AN :EL405 (1466) ( 8 l ( Continued )

(d) both (a) and (b) above.

(xvz) The magnetising current in a transformer is rich in

(a) third harmonic.

(b) fifth harmonic.

(c) seventh harmonic.

(d) thirteenth harmonic.

(xviz) Transformer transforms

(a) voltage.

(b) frequency.

(c) current.

(d) power.

(xviiz) The efficiency of a well designed transformer may be expected in the range of

(a) 75% to 80%.

(b) 65% to 70%.

(c) 85% to 90%.

(d) 95% to 99%.

(.xix) The leakage flux in a transformer depends upon

(a) supplyfrequency.

(b) load current.

(c) mutual flux.

(d) None of the three above.

S'l2: 5 AN :EL 405 (1466) ( 9 ) (Turn Over)



33

(xx) The shunt motor has

(a) hard torque-speed characteristics.

(b) soft torque-speed characteristics.

(c) speed varies with load.

(d) speed varies greatly with load.

)'12: 5 AN :EL 405 (1466) ( !(}) AG-1600



34

W'12: 5 AN: EL 405 (1466}

ELECTRICAL MACHINES

lime : Three hours

Maximum Marks : 100

Answer FIVE questions, taking ANY TWo from Group A, ANYTWOJrom Group Band ALL from Group C.

All parts of a question (a, b, etc. ) should be answered at one place.

Answer should be brief and to-the-point and be supplemented with neat sketclies. Unnecessary long answer may

result in loss of marks. ·


Figul'f!S on the right-hand side margin indicate full marks

Group A

1. (a) Explain the procedure of bringing a d.c. shunt generator in parallel with a loaded d.c. generator and transferring part load to it. 6

(b) A shunt generator, which gives a terminal voltage of 400 Vat no load and 360 V when delivering 100 A, is working in parallel with one which gives 400 Von no load and 350 V when delivering 100 A. If the voltage characteristics of the machines are approximately linear, find the common terminal voltage and the current in each when they are storing a total load oflOOA. 6

(c) Draw the circuit of' diverter field control' method of speed control of a d.c. series motor. Deduce a



35

suitable expression for motor speed in tenns of various resistances and constants of the machine. being negligible. The motor is fed from infinite bus at 8

3.3 kV. Its field excitation is adjusted to result in unity 2. (a) What does Dy 1 connection of a three-phase trans- power factor operation at rated load. Find the

former mean? Draw the windings, show the connec- maximum power and torque that the motor can tions labelling various terminals. Also, draw the deliver with its excitation remaining constant at this phasor diagram of voltages on high voltage side and value. 10 the low voltage side for this connection. 7

Group B (b) Where are star/delta and star/zig-zag connections

of three-phase transformers used ? Discuss. 7 5. (a) Explain the production of air-gap rotating field when

(.-.\ Th'-' n.n,go.n, rlo.lte'lo 1"""-Y'\Y'\o.,......+~--n 1-.o.c. 'QO£ ..,..n;.;._,..... ...... + .. t.n. the symmetrically wound armature of a three-phase ~n~l.;~~ ;n n .. ~~1;n,l -r-n~ n l.n1n-nnrl +1.-nn -l.n~n

\'-'I ..LU.V V}-'""'11 \ .. U,,.,.l\.U. '-'V.U.U'-'"""\lV.l.l J.~UL> -'U/0 JaL.JlJO, V.l llJ.\..( uta\.<uut'-' • .., .,uppuo;;;u uvtu a uaJauvo;;;u uuo;;;o;;;-pua::.o;;;

normal delta/delta connection rating. Explain. 6 supply. 8

3. (a) Explain the 'preventive reactor' on load tap changing (b) If the emf in the stator of an 8-pole induction motor arrangement of transf(mners. 6 has a frequency of 50 cis, and that in the rotor

(b) Considering that the resultant flux in the air gap of a 1 ~ cis, at what speed is the motor running and

synchronous generator is the phasor sum ofthe field what is the slip? 4 flux phasor and the armature reaction flux phasor, draw the phasor diagram showing various fluxes and (c) Draw the equivalent circuit of a three-phase indue-em( 6 tion machine and the torque slip characteristics for

a slip range of -1.0 to 2.0. 8 (c) If a field excitation of I 0 A in a certain alternator

gives a current of 150 A on short-circuit and a termi- 6. (a) Explain the auto-transformer starter for starting a oal voltage of900 Von open-circuit, find the internal three-phase squirrel cage induction motor drawing voltage drop with a load current of 60 A. 4 a circuit diagram. Find the ratios of starting to normal

(d) What is understood by direct-axis reactance and torques and currents as a function of transformer tap.l 0

quadrature-axis reactance of a synchronous (b) The power input to a three-phase induction motor machine? Which one of the two is smaller and why? 4 is 60 kW. The stator losses total! kW. Find the total

••• :4-1.. 4-1..- 1..-l- -C-1..-.--.- ,l; ___ n_/_'\ 4-l..n mechanical power developed, and the rotor copper

" '-" T:'---1-: .. t _ ______ __ 1 _ ___ ~.£- ...... t _ _ ~- _...._ __ ~- -----~-- __ .~... "1n/ -1~--

4 ... \_U) DXJ1H1111, W llll Ult:: llCill Ul fJili:tMJl Uh1~1 aUl\_i) j, Ult:: Joss per pnase, u tne niutur IS rumnng at ..)7o sup. V curves of a synchronous motor. 10

(c) State the salient features of a d. c. servo motor. 6 (b) A 1000 kW, 3-phasc, star-connected, 3.3 kV,

24 pole, 50 Hz synchronous motor has a synchro- 7. (a) Describe the frequency control method of speed no us reactance of 3.24 Q per phase; the resistance control of induction motors. Also, develop the

torque-speed relation under constant vif condition.12



36



37

5'13 : 5 AN : EL 405 (1466)

ELECTRICAL MACHINES

Time : Three hours

Maximum Marks : I 00

Answer FIVE questions, taking ANY TWO from Group A, ANY TWO from Group Band ALL from Group C.

All parts of a question (a, b, etc. ) should be answered at one place.

Answer should be brief and to-the-point and be supplemented with neat sketches. Unnecessary long answer may

result in loss of markS.



Group A

1. (a) Explain the parallel operation of d.c. shunt genera-tors. 6

(b) Two d. c. shunt generators are operating in parallel. One has an external characteristic which may be considered a straight line rising from 450 Vat noload to 500 Vat 100 A. The other has characteristic which may also be considered a straight line with 450 Vat no-load to 480 Vat 80 A. Determine the common bus bar voltage and current supplied by each machine when the total current supplied is 160A. 8

(c) Describe in detail the speed control of d.c. shunt motors by flux control method. 6



38

2. (a) What is meant by three-phase transformer vector 8. (a) Explain in detail the construction ofthe circle dia-groups ? What is the significance ofthe groups ? ,5 gram for induction motor, and the parameters

' ' determined for the circle diagram. 10 (b) Explain in detail the conversion of three-phase to

two-phase transformer. 10 (b) Explain the load test on three-phase induction

5 motor to evaluate its performance with a neat

(c) What are different transformer corrections ? sketch. 10

3. (a) What are the drawbacks ofbrake test on d.c. rna- Groupe chines? 5

9. Answer the following in brief: 10 X 2 (b) Explain in detail about Swinburne's test or No-

load test of d. c. motor. 10 (l) Why is the transformer rating in kVA?

(c) Write a short note on 'load tap changing'? 5 (iz) What is meant by leakage flux in general ?

4. (a) What are V-curves with regard to synchronous (iii) Write the necessity of damper winding in synchro-motor? Also, explain the same with practi~al signifi- nous machine. cance. 10

(iv) Which part of the d.c. machine is laminated and (b) Explain how a synchronous motor may act as a why?

synchronous condenser and draw the phasor dia-gram of the synchronous condenser. 10 (v) What is the difference between lap winding and

wave winding? GroupB

(vi) What is the design change made in an a.c. servo-r::. { ~\ n..-n"ITJ' f-ho. ..-.1~- f-£"\. .... r'l .. "fO ,...t,.,,........_... ....... O_;._..,+-= ........... rt..f" .rlr\."'lhl..o. roort.rtO. _,-+-.:...+.no -.h+n.;..., 1~-on-+--/'ll"'"t.t"'t. nl~- ..... J...n......,.r•i~n....:~.t-; ......... C) .Jo \UJ . .Lnaw LH'-' o:>up-LVl'-jU'-' vuruavL'-'llo:>Uvo:> VJ. uvuuu.rva0 "' lllV\.Vl LV UU\.allllllJ~aJ. I,..Vl\.fUY-~111' \.llla.ta\.IL~li;:,U\.1.., !

squirrel cage motor. 10 (vii) Define slip of an induction motor.

(b) Explain in detail .various methods of speed control for induction motors. 10 (viil) Why is the compensating winding provided in a.c.

series motor? 6. Explain working of various starters used for induction

motors with a neat sketch. 20 (ix) Explain the use of slip ring and brushes.

7. (a) What are various types of single-phase induction (x) Derive the condition for maximum torque. motors ? Discuss their respective applications. 6

(b) Explain in detail about the working principle and operation of split phase induction motor. 14



39

W'13: 5 AN: EL 405 {1466)

ELECTRICAL MAClllNES

Tzme : Three hours

Maximum Marks : 100

Answer FIVE questions, taking ANY TWO from Group A, ANY TWO from Group Band ALL from Group C.

All parts of a question ( a, b, etc. ) should be ans"rVered at one place.

Answer should be brief and to-the-point and be supplemented with neat sketches. Unnecessary long answer may result in

loss of marks.

1.



Group A

(a) Write the conditions to be satisfied before two d. c. generators are connected in parallel.

(b) Why is it necessary to provide equaliser bar for parallel operation of series generators?

(c) Two shunt generators, each with an armature resis-tance ofO.Ol Q and field resistance of20 n, run in parallel and supply a total load of 4000 A The emfs are 210 V and 220 V, respectively. Calculate the bus bar voltage and output of each machine.

2. (a) What are the advantages of a Transformer Bank of3 single-phase transformers over a three-phase unit transformer?

6

6

8

6



40

(b) Where are delta-star £onne_cti()n of a three-phase Group B transformer used? Show1hewindingconnection fora

DY"11 connection (for3~ lead). · 6 Startiitg from the equa~ion for the rotor current, show s. (a)

(c) How is a three-phase supply converted to a two- how you can get the equivalent circuit of a three-phase system? Show the connection, mentioning in phase induction motor. How is the mechanical power brief the winding arrangement and ~e power output.. 8 developed and calculated from the equivalent circuit? 8

3. (a) With the help of a phasot diagram, show how the (b) How do you estimate the, starting torque Tst and the regulation of an alternator is computed from the maximum torque T max from the parameters of the synchronous impedance of the machine for a given equivalent circuit? 4 load current. 6

/I."\ A three-phase star connected 1000 kVA, 11 kV (c) A 6-pole, 50 Hz, three-phase induction motor develops

~U) a usefui torque of 150 N-m when the rotor cw:rent alternator has rated current 52.5 A. The a.c. resis- makes 120 complete cycles/min. · tance of the winding/phase is 0.45 Q. The test result shows that a field current of 12.5 A gives an open- (z) Calculate the slip, mechanical power developed circuit voltage of 422 V between lines and 52.6 A of and net shaft power developed, if the torque armature current on short-circuit. Determine the full lost in friction is 8 N-m. 2+3 load voltage regulation of0.8 p.f.leading. 8

(c) Draw a simple circuit diagram for an alternator to be (iz)Detennine the copper loss and the input, if total

stator loss = 800 W. · 2+1 synchronised with-the bus bar. Write the steps to be followed briefly ... 6 6. (a) Why is star delta starting preferred to DoL (Direct

on Line) starting in case oflarge induction motors? 4. (a) When do you get two different reactancesXd andX in Compare the relative values of starting torques in two ·

a synchronous machine ? Hpw do you determine cases. 4+4

these reactances experimentally? Show the connection (b) Show the circuit diagram for a star-delta starter. 6 and write the steps to be followed. 8

(c) N arne different methods of controlling the speed of an (b) Draw the phasor diagram .of a round rotor synchro- induction motor. Describe any one of the methods

nous motor with the field winding excited to more with the diagram of the control element. 2+4 than 100%. 4

7. (a) Describe how from a pulsating magnetic field in a

(c) A star-connected synchronous withXs = 1 .. 2 Wphase single-phase motor, a rotating magnetic field is created

is connected to a 400 V, three-phase supply and with the help of an auxiliary winding. 8

takes a current of 20 A at 0. 8 power factor leading. (b) Show how .an armature winding connected with a Estimate the e~citation current in terms of its nominal 8

value. series field can develop a non-zero average torque,. even if the supply is a.c. 4



41

(c) Sketch the torque-speed characteristic of a singlephase induction motor showi~g the forward, back-ward and net torque in the speed range of g

-N~ s;n::s; N.

8. (a) Explain, with a neat diagram, how a torque is produced in a shaded pole induction motor by the splitting of the flux. 8

(b) How can one use a two-phase motor for measuring the speed of a shaft? Show the schematic diagram. 6

(c) Explain, with a neat diagram, the arrangement of windings in a two-phase a.c. servomotor. Show the torque-speed characteristics and mention its charac-teristic feature relating to the rotor impedances. 6

Group C

9.· Answer any five of the following: 5x4

(1) Show the connection for two identical d.c. series motors for traction purposes at low speed and high speed.

(il) Mention the advantages of a V-V (open delta) connection of a three-phase transformer, showing the connection.

(iil) Explain the need for interpole and compensating windings in a d.c. machine.

(iv) How can one control the speed of a d.c. shunt motor?. Show the simple connections. .

(v) If a transformer llkV/400V, 50 Hz is energised by a source of 11 kV, 60 Hz supply, will the no-load losses increase or decrease or remain unaltered ? Justify your answer.

(vi) Explain the principle of operation of a d.c. servomotor.

(vii) How are the direct axis and quadrature axis reac~ces,~ ~dXq' determined experimentally?

(viiz) What are the advantages of a three winding transformer ? Show its connection and equivalent circuit

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